The United States has formally complained to China after alleging that the Chinese military injured two U.S. airmen by directing high-grade lasers at American aircraft in Djibouti.

Pentagon spokeswoman Dana W. White said at a briefing on Thursday that the United States has requested China investigate incidents in recent weeks in which U.S. aircraft in Djibouti have been affected by unauthorized Chinese laser activity.

White said the Pentagon was confident that Chinese nationals were responsible. She said there had been more than two but fewer than 10 such incidents, which she said had increased in frequency in recent weeks.

The United States and China both have bases in Djibouti, an East African nation on the Gulf of Aden with fewer than 1 million people. Djibouti has become a hub of foreign military activity in recent years, hosting bases from an array of countries including France, Italy and Japan. Saudi Arabia is planning to build a base there as well.

Camp Lemonnier, a U.S. naval expeditionary base in Djibouti that is home to about 4,000 American service members, serves as a hub for American counterterrorism activities in nearby countries such as Somalia and Yemen. The United States also has been running drone operations out of Djibouti.

The incidents with the lasers, which can temporarily blind pilots, come as one of the first major dust-ups since China opened its first overseas military base in Djibouti in 2017. The facility’s opening raised concerns among American military officials about the proximity of the Chinese military installation to American forces. The Chinese have characterized the base as a logistics hub for peacekeeping and anti-piracy operations.

American authorities recently issued a notice warning pilots that there had been incidents involving “a high-power laser” near the Chinese base in Djibouti. The notice urged pilots to “use extreme caution when transiting near the area.”

White said two American airmen had suffered minor injuries but didn’t provide details. According to CNN, two members of a C-130 aircrew suffered minor eye injuries because of exposure to military-grade lasers.

IHS Jane’s Defence Weekly, citing “multiple intelligence sources,” reported in April that the Chinese navy is suspected of operating a high-powered laser weapon at the base in Djibouti or on a ship offshore.

“The use of lasers to temporarily blind pilots has been increasing over the years and dates back to the Cold War when U.S. Navy pilots were periodically attacked by lasers emanating from Soviet naval vessels and spy trawlers,” Jane’s said.

A spokesman for the State Department declined to comment on the contents of its communications with China.

In a regular briefing at China’s Foreign Ministry, spokeswoman Hua Chunying dismissed the U.S. complaints.

“After careful verification we have told the United States explicitly that the accusation is totally inconsistent with fact,” she said.